Electrical energy is storable with the aid of batteries. Batteries convert chemical reaction energy into electrical energy. In this context, it is differentiated between primary batteries and secondary batteries. Primary batteries are for one-time use only, while secondary batteries, which are also referred to as rechargeable batteries, are rechargeable. In this case, a battery includes one or multiple battery cells.
So-called lithium-ion battery cells are in particular used in a rechargeable battery. Battery cells of this type distinguish themselves by high energy densities, thermal stability, and an extremely low degree of self-discharge. Lithium-ion battery cells are used, inter alia, in motor vehicles, in particular in electric vehicles (EV), hybrid electric vehicles (HEV) as well as in plug-in hybrid electric vehicles (PHEV).
Lithium-ion battery cells have a positive electrode, which is also referred to as a cathode, and a negative electrode, which is also referred to as an anode. The cathode and the anode each include a current collector onto which an active material is applied. The active material for the cathode may, for example, be a metal oxide. The active material for the anode may, for example, be a graphite or a silicon.
Lithium atoms are stored in the active material of the anode. During the operation of the battery cell, i.e., during a discharging process, electrons flow in an external circuit from the anode to the cathode. During a discharging process, the lithium ions migrate within the battery cell from the anode to the cathode. An anode material, such as graphite, is able to store lithium ions in its crystal lattice if their charge is compensated for by one additional electron per lithium ion in the graphite. During a discharging process, the lithium ions are then expelled again, which is also referred to as deintercalation. During a charging process of the battery cell the lithium ions migrate from the cathode to the anode. During the process, the lithium ions are reversibly integrated into the active material of the anode, such as graphite, which is also referred to as intercalation.
The electrodes of the battery cell are designed in a foil-like manner and wound to form an electrode coil by inserting a separator in-between which separates the anode from the cathode. This type of electrode coil is also referred to as a jelly roll. The two electrodes of the electrode coil are electrically connected with the aid of collectors to the poles of the battery cell, which are also referred to as terminals. A battery cell generally includes one or multiple electrode units. The electrodes and the separator are surrounded by an electrolyte which is generally liquid. The electrolyte is conductive for the lithium ions and makes the transport of lithium ions between the electrodes possible.
The battery cell furthermore includes a cell housing which is made of aluminum, for example. The cell housing is generally prismatic, in particular cuboid-shaped, as well as pressure-resistant. The terminals are located outside of the cell housing. After connecting the electrodes to the terminals, the electrolyte is filled into the cell housing.
A generic electrode, in particular an anode, for a lithium-ion battery is described in U.S. Patent Appl. Pub. No. 2013/0288126 A1, for example. The active material of the anode contains silicon as well as an ionically conductive polymer, such as triethylene oxide.
An active material for an anode including a binder which is made of three polymers in addition to silicon and which increases the ionic conductivity of the active material is described in U.S. Patent Appl. Pub. No. 2010/0129704 A1.
U.S. Patent Appl. Pub. No. 2014/0045065 A1 describes an active material for an anode is described which contains a polymer matrix made of electrically conductive polymers and in which silicon is stored.
Another active material for an anode is described in PCT Appl. No. WO 2013/116711 A1. The active material includes a conductive polymer which is coated with silicon.
As an active material of the anode, silicon has an increased storage capacity for lithium ions as compared to graphite. The silicon as the active material of the anode is, however, exposed to attacks from chemical reactions of the silicon with the liquid electrolyte in some electrolytes. The reaction products, together with the lithium contained in them, then accumulate on the surface of the active material, where they form a layer which is referred to as “solid electrolyte interphase” (SEI). Since the silicon does not participate in the back reaction, the lithium, which accumulated there, is no longer available for intercalation and the transport of lithium ions between the electrodes. The capacity of the anodes for lithium ions continuously decreases and the cathode is then no longer able to get fully charged.